(1) Field of the Invention
The present invention relates to an instruction executing control system controlled by a microprogram in an information processing equipment, particularly to a microcode reading control system with an advanced control.
(2) Description of the Realted ARt
A machine instruction in an information processing equipment is usually executed by the following steps. Namely, the machine instruction is read from a main storage in response to an address indicated by an instruction counter, the maching instruction is decoded, various controls are exeucted corresponding to the decoded instruction, and the instruction counter is increased by a length of the machine instruction.
Since the machine instructins are stored in the main storage device in the processing sequence, when the machine instructions are read, an address of the next machine instruction to be read can be anticipated.
Therefore, while the equipment is decoding and processing, at the same time the equipment can read the next machine instruction. That is, an advanced control operation is possible.
However, if a branch instruction is exeucted, since the content of the instruction counter is refreshed or overwritten, a problem arises in that the fetch of the instruction during advanced control operation becomes difficult.
The branch instruction is important and occurs frequently in the information processing equipment, and therefore, high speed processing of the branch instruction would greatly contribute to an improvement of the equipment performance.
Further, the information processing equipment wherein processing of a machine instruction is divided into steps, each to be executed by a microcode, and the process in each step is carried out by a pipeline process in a multi-stage manner, an advanced read of the microcode corresponding to the advanced reading of the machine instruction can be made.
Therefore, an effective microcode reading control system must be provided in the advanced reading control system when a branch instruction is processed.